When you use cache mode outlook will save local copy of the GAL (OAB)
You can update the the Offline Address Book manually from EMC but it will take some time for all the workstations to get the update because by default the outlook will retrieve the OAB every 24 hours, you can of course guide the users to do it manually.
Read more here:http://www.howto-outlook.com/howto/oabupdate.htm